The present invention relates to a hydraulic clamp for securing a workpiece to a work-holding plate such as a table or work-pallet of a machining center. More specifically it relates to a hydraulic clamp of a clamp arm and a hydraulic cylinder arranged in a clamp housing one before the other longitudinally so that the clamp arm is swung longitudinally by the hydraulic cylinder.
This invention pertains to an improvement of a prior art machine whose basic construction is as shown in FIGS. 11 through 13. FIG. 11 shows a prior art counterpart described in Japanese laid-open utility model application No. 191227/84, and FIG. 12 and 13 show another prior art of which the present inventor is aware. With regard to these references art counterparts 1 and 2, parts and members having like functions are referred to by like numerals.
Referring to FIGS. 12 and 13, a hydraulic clamp 202 comprises a clamp arm 204 and a hydraulic cylinder 205 arranged one before the other longitudinally in a clamp housing 203, the clamp arm 204 is pivotally held by a fulcrum shaft 209 extending from both walls 207, 207 of the clamp housing 203 to be freely swingable longitudinally. An elastic body 257 is to push back the clamp arm 204 to be inward of the front face of the clamp housing 203, while it is driven outward of the front face of the same when a piston 214 of the hydraulic cylinder 205 is driven forward.
The effective of this basic construction is described below under reference mainly to FIG. 12 and FIG. 13. The clamp housing 203 has its front and rear ends secured to a work pallet 201 by means of plurality bolts 237, the clamp arm 204 is swung forward by hydraulically driving forward the piston 214 of the hydraulic cylinder 205, a workpiece W (or die) is pressed by the clamp arm 204 against a work holder 201a (as shown in the figure by a two-dot chain line) and the workpiece W, thus clamped, is worked on or work using a die is carried out. When the work is over, the clamp arm 204 is swung back by the elastic body 257 into the clamp housing 203 automatically as the piston 14 is retracted.
With the clamp in the above-described condition, an inverting moment is caused to push up the clamp housing 203 with its rear end edge as fulcrum by the clamp reaction force R applied by the workpiece W (or die) to the clamp arm 204. The fastening force of the above-mentioned bolts 237, however, offsets this inverting moment and the clamp housing 203 is held on the work pallet 201.
According to the prior art 1 FIG. 11, the above-mentioned construction had means for pushing back the clamp arm 204 arranged as follows. In the clamp housing 203 a spring room 261 is formed below the hydraulic cylinder 205, an elastic body 257 consisting of a compressive coil spring is housed therein and the springy force of the elastic body 257 is transmitted to the clamp arm 204 via a link mechanism 262.
According to this prior art, however, the height of the clamp housing 203 is increased by that of the spring room 261 to make the hydraulic clamp bulky and, worse, the required link mechanism causes an increase of the number of the constituent parts to thereby make the composition of the hydraulic clamp 202 more complicated.
In the basic construction of the prior art see FIGS. 12 and 13, the elastic body for pushing back the clamp arm 204 is formed as follows. The elastic body 257 consists of a torsion coil spring, a through notch 258 is provided near the lower end of the clamp arm 204 for housing therein the spring extending axially with respect to a fulcrum shaft 209, in the notch 258 the fulcrum shaft 209 has set thereon the elastic body 257 consisting of the torsion coil spring with one end thereof held by the clamp housing 203 and the other end thereof connected with the through notch 258.
The above-described construction of this prior art, which will eliminate or solve the problems of the earlier described prior art per FIG. 11, has the following defects.
(1) Debris such as cutting chips is allowed to easily get into the clamp housing 203 through the notch 258 for housing the spring during working of the workpiece W, and such debris may find its way onto the back of the clamp arm 204 or the sliding face of the piston 214 to cause malfunction thereof;
(2) since the point of application of force of the elastic body 257, i.e. the other end thereof 257b, is located in the vicinity of axis of the fulcrum shaft 209, the arm length of the moment of the push-back force of the elastic body 257 is short, hence it is impossible to powerfully push back the clamp arm 204 at the time of unclamping;
(3) although the clamp reaction force R applied by the workpiece W of the clamp 204 during clamping is held by the fulcrum 209 of the clamp arm 204, the clamp arm 204 has formed therein the notch 258, hence the force applied by the clamp arm 204 to the fulcrum shaft 209 acts near either axial end of the fulcrum shaft 209. Accordingly, the bending moment and/or shearing force acting on the fulcrum shaft 209 are/is caused to increase, hence it is required to increase the diameter of the fulcrum shaft 209, and this results in making the hydraulic clamp 202 bulky.